Coach Screw Sizes for Timber Fixings | Hebei Wotu Fastener Manufacturing Co., Ltd.
Coach screw sizes should be chosen around the timber thickness, bracket hole, expected load direction and the amount of thread that can bite into the base material. The wrong size can split timber, bottom out before clamping, or leave too little engagement for a reliable fixing.
For purchasing, the key decision is not only diameter and length. Head type, thread form, finish, pilot hole practice and the material being fixed all change which coach screw specification is suitable.
Brand: Hengzuan Surface: blue and white zinc plated Specifications: in stock ST2.9/ST3.5/ST4.2/ST4.8/ST5.5 Chinese national standard full series, complete rod length 13~60mm, supports nonstandard length customization; drilling adapts to steel sheet thickness 2~6mm Packaging: 25kg cartons, whole wooden pallets
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Brand: Wotu Surface: Natural black oxide treatment, customized hot-dip galvanized, zinc plated as required Specifications: Follow Chinese national standard GB/T799-1988, in stock M12-M48, total length 120mm~1500mm, nonstandard customized diameter, bending length, total screw length available Matching: Nuts, flat washers and spring washers of the same specification can be supplied in complete sets. Packaging: Bundling and packaging, using wooden pallets and film-wrapped export packaging in large quantities
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Brand: Hengzuan Surface: Color zinc plated, optional blue-white zinc plated, black oxide treatment Specifications: Chinese national standard ST4.2/ST4.8/ST5.5 specifications, rod length 13~50mm in stock, supports nonstandard length customization Packaging: 25kg export cartons, full wooden pallets wrapped with film, supports neutral packaging, private-label customization
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Brand: Hengzuan Surface: blue and white zinc plated, can be customized color zinc plated, black oxide plated Specifications: Comply with GB/T15856.2-2002 Chinese national standard, in stock ST4.2/ST4.8/ST5.5 full range of specifications, length 13mm~75mm optional, supports customized nonstandard rod lengths; drilling adaptation plate thickness 0.7~6mm Packaging: 25kg export cartons, full pallets wrapped with film, supports neutral export packaging, customer private-label customization
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Brand: Hengzuan Surface: blue and white zinc plated, can be customized color zinc plated, black oxide plated Specifications: in stock 3.5/3.9/4.2/4.8/5.5 mainstream wire diameter, rod length 13mm~75mm full size range, nonstandard length can be customized as needed Packaging: 25kg export cartons, full wooden pallets wrapped with film, supports neutral export packaging, private-label customization
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Brand: Hengzuan Surface: color zinc plated, customized blue-white zinc plated, black oxide Specifications: in stock 3.9/4.2/4.8/5.5 full diameter, length 13mm~75mm full size range, supports customized nonstandard rod lengths Packaging: 25kg export cartons, full pallets wrapped with film, supports neutral export packaging, customer private-label customization
Read MoreCoach screw selection notes
Useful coach screw selection starts with diameter, length, head style, thread length, coating and the material being fixed. Buyers should also confirm the hole size in the bracket or plate, because the screw shank and head must fit the hardware as well as the timber.
| Criterion | What To Check | Why It Matters | Buyer Tip |
|---|---|---|---|
| Diameter and length pairing | Confirm the nominal diameter and overall length, including the thread length if it is not fully threaded. | A coach screw that is too short gives weak timber engagement; one that is too large can split the wood or overload a bracket hole. | What timber thickness, fixture thickness, and required embedment should the coach screw size match? |
| Thread form for timber holding | Compare coarse timber threads, shank length, and whether the screw has a gimlet point or requires a clean pilot hole. | Thread geometry affects pull-out resistance, driving effort, and the risk of cracking dense or dry timber. | Will the screws go into softwood, hardwood, engineered timber, or masonry with a plug? |
| Pilot hole requirement | Ask for recommended pilot hole guidance for the selected diameter and base material. | Coach screws usually need pilot holes; the wrong hole can reduce holding power or make installation difficult. | Can you provide pilot hole advice for this screw size and the buyer’s timber species? |
| Head and driving clearance | Verify hex head dimensions, washer compatibility, and tool access around the fixing point. | The head must seat properly on brackets, plates, or washers, and the installer needs enough space for a socket or spanner. | Will the screw be used with a washer, and is there clearance for the intended driving tool? |
| Surface finish for the environment | Match the coating or material to indoor, outdoor, damp, or chemically exposed use, and confirm compatibility with connected materials. | Finish choice affects corrosion resistance, appearance, and service life, especially in exterior timber or treated wood. | Is the application indoors, outdoors, near moisture, or in contact with treated timber? |
The practical difference between coach screw sizes is how much clamping force and thread engagement they can provide without damaging the workpiece. A larger diameter gives more holding area and suits heavier brackets or structural timber connections, but it also needs a larger clearance hole and a more careful pilot hole to reduce splitting.
Length matters in a different way. A coach screw must pass through the fixture and still leave enough threaded section in the timber or substrate. If the screw is too short, the fixing may feel tight without having enough grip. If it is too long, it can break through the far side, hit hidden hardware, or bottom out before the head seats properly.
Start with the joint, not the screw table. Check the thickness of the bracket or plate, the timber section, and whether the load pulls outward, hangs downward, or resists shear. A fence post bracket, timber frame connector and machinery base fixing may all use coach screws, but they do not need the same diameter or length.
Match the head and drive to installation access. Hex head coach screws suit spanners, sockets and higher installation torque, especially where the fixing sits outside the finished surface. If the head must sit flush or under a cover, confirm whether a different fastener style is more appropriate instead of forcing a hex head into a recessed detail.
Do not ignore the pilot hole. Coach screws are usually installed into pre-drilled timber, and the pilot hole affects both splitting risk and holding strength. A common mistake is using the same pilot approach for softwood, hardwood and engineered timber. Dense material generally needs more care because excessive driving torque can damage the screw, the head, or the surrounding timber.
Finish should follow the environment. Indoor dry timber may not need the same corrosion resistance as outdoor fencing, decking, agricultural structures or treated timber. Before ordering, confirm the screw diameter, overall length, head size requirement, material or coating, packing preference and the base material so the quotation matches the actual application.
| Criteria | Importance | Description | Example Options |
|---|---|---|---|
| Nominal diameter | Important for thread engagement, load demand and splitting risk. | Choose a diameter that fits the bracket hole and provides suitable thread area without removing too much timber or leaving inadequate edge distance. | A lighter bracket may need a smaller diameter, while a substantial timber connector may justify a larger one if the timber section supports it. |
| Overall length and threaded engagement | Critical for clamping the joint without breakthrough or bottoming out. | Account for the fixture thickness, any washer or spacer, and the length that must enter the timber. The usable engaged length is more relevant than overall length alone. | A thick metal bracket requires a longer screw than a thin strap fixing on the same timber member. |
| Pilot and clearance holes | Important for preventing splitting and achieving consistent installation. | The clearance section should allow the screw to pull the fixture tight, while the pilot section should suit the screw and base material. Softwood, hardwood and engineered timber may need different hole preparation. | A dense hardwood assembly may need more careful pilot-hole control than a softwood assembly of similar thickness. |
| Head and drive style | Important where access, seating and the finished surface affect installation. | Hex heads work with sockets or spanners and remain accessible above the surface. A recessed or flush detail may call for another fastener style rather than forcing a hex head into the joint. | An exposed post bracket can accommodate a socket-installed hex head, while a covered furniture detail may need a flush-seating alternative. |
| Material, coating and exposure | Important for outdoor, damp or treated-timber applications. | Select the material or finish according to moisture, weathering and contact conditions, then confirm compatibility with the timber and any attached metal hardware. | A dry indoor assembly and an outdoor treated-timber connection may require different corrosion-resistance considerations even when their dimensions are similar. |
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Add the bracket or plate thickness to the amount of threaded engagement needed in the timber, while leaving room for the head to seat fully. Check the assembled joint so the tip will not break through the timber, strike hidden hardware or bottom out before clamping.
A larger diameter can provide greater thread area and suit heavier brackets or higher loads, but it also requires a larger hole and can increase the risk of splitting. The timber section, load direction, edge distance and installation method matter as much as nominal diameter.
The clearance hole passes through the bracket or upper timber layer so the screw can pull the parts together, while the pilot hole guides the threaded section into the timber. Treating both holes as the same size can reduce clamping or increase splitting, especially in dense hardwood and engineered timber.
A coach screw is convenient where access is available from one side and the base material can hold its threads. A through-bolt with a nut may be more suitable when both sides are accessible, the joint may be repeatedly dismantled, or the connection requires a different clamping arrangement.
State the nominal diameter, overall length, head and drive style, thread form or threaded length if important, material or finish, quantity and packing preference. Also identify the timber or substrate, bracket-hole size, indoor or outdoor exposure and whether treated timber is involved so the proposed dimensions can be checked against the application.
Please share the required diameter and length if known, the timber or substrate material, and whether the screws will be used indoors, outdoors or with treated timber.
